Patent GB-481731-A (GB)
Title: Improvements in or relating to thermionic valve amplifiers
IPC: H03G5/26
Filed: 1936-06-15
Abstract: 481,731. Valve circuits for wireless reception. ELECTRIC & MUSICAL INDUSTRIES, Ltd. June 15, 1936, Nos. 16632 and 16633. Convention dates, June 15, 1935 and May 5, 1936. [Class 40 (v)] The selectivity of a radio receiver is varied inversely as the ratio between the amplitudes of the desired and adjacent interfering carrier waves. Circuits are also described for varying the frequency response of the L.F. amplifier in accordance with this ratio. In one embodiment, Fig. 1, a a receiver comprises a radio frequency amplifier and frequency changer 2 feeding a moderately selective LF. amplifier 3, detector and L.F. amplifier 5 and reproducer 6. The circuit 2 also feeds a highly selective I.F. amplifier 4 feeding, an A.V.C. rectifier 10 and the opposed outputs of circuits 3, 4 also feed an aperiodic amplifier 8 and rectifier 9 which produces selectivity controlling voltages depending mainly upon the amplitude of frequencies adjacent the desired carrier. These voltages control damping valves 21, 31 in the circuits 2, 3 respectively and the A.V.C. voltages from rectifier 10 are applied to circuits 2, 3 and 4 and thus the selectivitv control voltages are a function of wanted and unwanted carrier amplitudes. A visual indication of the.amount of selectivity in use may be given by a neon tube, the length of the luminous column of which is varied by the selectivity controlling voltages. The latter may be kept within predetermined limits by a relay 89, Fig. 7, responsive to the anode current of a multielectrode valve 80, one grid of which rectifies the input from the interference signals and another of which is controlled by the A.V.C. rectifier 83. The relay 89 has a moving contact 92 co-operating with contacts 90, 91 connected to fixed sources of grid bias for the controlled valve 95. In the system shown in Fig. 10, the frequency response of the low-frequency amplifier 103 is modified according to voltages produced by the rectifier 102 to which are applied in opposition the outputs from a sharply selective amplifier 99 and a more broadly tuned amplifier 98 which also feeds the L.F..amplifier 103 and the A.V.C. rectifier 101 which controls amplifiers 98, 99 and 102 and the radio frequency preamplifier 97. The frequency response of the low-frequency amplifier may be controlled by the inclusion of a resonant circuit 111, 111<1>, Fig. 13, tuned, to 7000 cycles/see. and a resistance 110 of 2000 ohms in the anode circuit of the L.F. valve 108 which is shunted by a valve 107, the impedance of which is varied by control voltages from the source A.D.S. after amplification in a valve 105. Alternatively, the variable grid to plate capacity of a controlled valve may be shunted across the load resistance of a detector diode or a low-frequency amplifier. The selectivity and /or low-frequency response may be controlled by an electro-mechanical device. Thus the anode current of a valve 124, Fig. 15, is passed through the coil 133 proportionally to rotate a shaft 134 which varies the capacity of a coupling condenser 114 in the input band-pass filter 112, 113 and the resistance of a damping resistance 136 in the tuned circuit 135. The valve 124 is fed through an overcoupled band-pass filter 122, 123 and its output circuit comprises two opposed branches, a circuit 125 tuned to the I.F. and an aperiodic branch 126 having the same impedance to the wanted carrier, the coils 127 and 128 in the branches being coupled to a rectifier 130, the D.C. output of which controls the anode current of valve 124. A further winding 137 on the electro-mechanical regulator carries the anode current of A.V.C. controlled valves 115, 116. In a modification, Fig. 16 (not shown), the rotary shaft controls a reaction condenser, a condenser compensating the variations in sensitivity caused by the latter and a condenser controlling the frequency response of the L.F. amplifier. In a further embodiment, Fig. 17 (not shown), a highly selective amplifier, e.g. with a crystal filter, and a less selective amplifier have common input and output circuits and an automatically operated electiomechanical device varies capacities which vary the relative coupling between the amplifiers and the output circuit. A method of deriving A.V.C. voltages is described in which the amplifier 178, 179, 180, 180<1>, Fig. 21, feeds an auxiliary amplifier 181 coupled to a diode rectifier 185, the D.C. voltage from which controls the main amplifier valves. The amplification of the auxiliary valve is also controlled by a biassing voltage derived from the anode circuit of the controlled valve 178. Other Figures show circuits in which a voltage is derived to operate a visual indicator showing correct tuning and the degree of selectivity obtaining at any time. Specifications 408,531 and 417,284 are referred to.